This invention relates to the manufacture of two-piece golf balls.
The manufacture of two-piece golf balls, i.e., golf balls with only a solid core ball and a dimpled outer shell, has normally been done in an injection molding process. In this process, the solid core is first placed inside of a mold cavity, a set of pins then pushes upward through the bottom of the cavity to raise the core to a central position, and a molten material is then forced into the cavity and around the core to form the outer shell.
Although this process makes well-formed balls, it is a difficult one to control and requires skilled operators. Also, it requires expensive tooling and expensive maintenance to the tooling. Ideally, it would be preferred if a less expensive process could be found.
While manufacturers have had to use injection molding to form the dimpled shell on two-piece balls, they have been able to use a much less expensive process of compression molding for forming a similar shell on three-piece balls, i.e., the traditional golf ball with a small rubber core ball surrounded by elastic windings and then the outer shell. Obviously, the manufacturers would prefer to use compression molding in forming twopiece balls because the equipment and tooling are already available in their plants. Further, the general process of producing the outer shell would be less expensive because it would be easier to control and would therefore not require expensive, highly skilled operators.
Unfortunately, attempts to compression mold perfectly shaped, two-piece balls have failed because, unlike the wound centers of three-piece balls, the solid cores of two-piece balls are relatively incompressible. In three-piece balls the wound core has small voids between the latticework of elastic windings. During compression molding of the cover for a three-piece ball, the wound core is able to be compressed into a smaller volume because the elastic windings fill these voids. Since the windings seek to return to their original positions, this creates an internal "back pressure" within the molding cavity. This internal "back pressure" presses the cover stock for the shell firmly against the inside walls of the mold and results in the final product being an exact replica of the dimpled mold cavity. Conversely, since the solid core ball is relatively incompressible, it creates no appreciable internal "back pressure" during the compression molding of two-piece balls. On the contrary, the solid core material expands upon heating, and displaces cover stock from inside the mold. During the cooling cycle, the contents of the mold contract and pull away from the mold's sidewalls. Since there is no internal "back pressure", this pulling away causes distortion of the ball's surface and makes the product unacceptable.
In 1902, Eleazer Kempshall attempted to solve this problem by using telescopically mating dies to compression mold a dimpled cover for two-piece balls. As described in his U.S. Pat. No. 695,867, Kempshall's process of compression molding includes a pair of male and female dies. As shown in FIGS. 5-7 of the Kempshall patent, the female die has a rim or step C on which a small amount of the cover material extrudes when the dies are initially pushed together to form the cover. The overflow material is trapped between the approaching edges B and C of the two dies. Continued axial pressure on the dies causes the edges B and C to move toward one another and force the trapped material to flow back inside the mold cavity to create an integral dimpled cover for the ball.
While the Kempshall attempt was noteworthy, it has its drawbacks. As explained in the patent, the produced cover is not uniform because it has a welt at its equator. This welt prevents the ball from being consistently hit the same distance with any single golf club by an accomplished golfer. Since predictability in the length of a "shot" is important for a low score in golf, this lack of consistency and the welt which causes it are highly undesirable.
The inconsistency occurs when the "strike" of the golfer's club head hits against the welt. Because this portion of the cover is fatter than the rest, the strike creates less compression of the ball than it would if any other portion of the cover were struck. This lesser compression causes the ball to go a shorter distance than it would normally travel.
In addition to creating a welt, the compression molding process of Kempshall allows the size of the welt to vary from ball to ball. Since the volume of the solid core for two-piece balls can vary substantially from one core to the next, it is difficult to precisely control the amount of cover stock extruded from the mold cavity during mating of the dies. With the Kempshall mold, if a large core were encountered, an excessive amount of cover stock would be extruded and the same large amount would be forced back into the mold cavity with the result being an even larger deformation or welt at the center of the finished ball cover. Also, wear would occur on the approaching edges B and C of the dies because of the extra pressure placed on the edges by the increased bulk between them.
Accordingly, a need exists for an improved set of compression dies by which the deficiencies of the Kempshall dies can be overcome so that a consistently well-shaped dimpled cover for two-piece golf balls can be formed by compression molding.
It is therefore the primary object of the present invention to provide an improved set of dies by which well-formed dimpled covers can be compression molded for two-piece golf balls.
It is another primary object to provide a new mold for compression molding the cover onto two-piece golf balls wherein the mold has a unique valving system for preventing or minimizing the welt formed in golf balls manufactured by the Kempshall method.
It is a more specific object to provide an improved set of telescopic dies for compression molding two-piece golf balls in which the "Kempshall" welt is prevented by draining off excess extrusion during mating of the dies so that the amount of extruded material between them that is forced back into the mold cavity is only the amount precisely needed to produce a well-shaped dimpled cover.
It is yet another object to provide a set of improved dies that are economical in design, yet extremely durable and effective to use.
The above and other objects and advantages of this invention will become more readily apparent when the following description is read in conjunction with the accompanying drawings.